SPR is well known for the detection of chemical species. SPR may be achieved by using the evanescent wave which is generated when a polarized light beam is totally internally reflected at the interface between a dielectric medium, eg glass, and a thin layer of metal. The technique is described by Lieberg et al in Sensors and Actuators, 4, 299. The basis for the application of SPR to sensing is the fact that the .oscillation of the surface plasma of free electrons which exists at a metal-dielectric boundary is affected by the refractive index of the material adjacent to the metal surface. Resonance occurs when the angle of incidence of the radiation has a particular value, and this value is dependent on the refractive index of the material adjacent to the metal. Thus, changes in this refractive index give rise to changes in the angle at which resonance occurs. Resonance may be detected as a dip in the intensity of the internally reflected beam.
In a typical SPR device, the thin layer of metal is coated on the surface of a glass prism. Recently, European Patent Application number 0346016 (Amersham) has suggested that the sensitivity of an SPR device may be enhanced by using the phenomenon of long range SPR. Long range SPR is described by Quail et al in Optics Letters, 8, 377 and involves a thin metal layer isolated from a coupling prism by a low refractive index "spacer" layer. This extra layer allows excitation of an SPR resonance on both metal dielectric interfaces, top and bottom, as opposed to the single resonance seen when just a metal layer is used. If the metal layer is thin enough, the field profiles from the two resonances will overlap, and they will have the opportunity to interact, provided that certain conditions are met. In particular, the propagation constants k of the two resonances must match, or at least be close to matching. Interaction will produce an anti-symmetric and a symmetric combination of the fields, labelled long range and short range SPR respectively. The anti-symmetric combination has a greater proportion of its field outside of the metal layer, and so suffers less loss, travels further along the surface (hence the name), and has a narrower resonance. The symmetric combination travels a shorter distance along the surface, and has a broader resonance.
The condition for matching propagation constants has conventionally required the presence of dielectrics of the same refractive index at the two interfaces. This is a problem in sensors, eg biosensors, in which the sample under test is aqueous (blood plasma, for instance) and has a refractive index of approximately 1.33. Spacer materials having refractive indices close to this value are not common, and those which are available cannot readily be coated with a thin layer of metal. Magnesium fluoride has a refractive index of about 1.38, which is close enough to show coupling, but is some way from optimal.
We have now devised a long range SPR device in which the above mentioned problem is overcome or substantially mitigated.